1. Field of the Invention
The present invention concerns density separation for use in digital multi-density printing. More particularly, the invention is directed to an efficient technique for separating an input signal representing a desired intensity level for a certain color into two signals: one corresponding to a desired quantity level of low optical density ink of that color and the other corresponding to a desired quantity level of high optical density ink of the same color.
2. Description of the Related Art
Recently, it has been proposed to perform digital printing using multiple inks, each having a different optical density but the same color, for each primary color. For example, low density cyan ink will print a lighter cyan dot than high density cyan ink. By providing the option of intermediate values for each dot on a page, multi-density printing can often provide better resolution and/or color range. One such conventional technique thus uses both high and low density inks for each of cyan (C), magenta (M), yellow (Y) and black (K).
An example of conventional processing for multi-density printing is illustrated in FIG. 1. In step S101, multi-value data for each of the red (R), green (G) and blue (B) components are input and then color corrected based on input device characteristics to obtain multi-value CMY data. In step S102, undercolor removal and black component generation are performed to extract a black component from the multi-value CMY data. In step S104, color correction is performed based on factors such as type of output medium to be used. In step S105, the CMYK values are output corrected to compensate for factors such as printer head characteristics and printing direction.
Up to this point, the data has been processed in multi-value format, typically with each color component represented by a value ranging from 0 to 255 (8 bits). In step S106, these multi-value data are converted to binary patterns for the high density inks (shown in FIG. 1 as CMYK) and the low density inks (shown as in FIG. 1 as cmyk) using multi-level halftoning. Thus, density separation in this particular conventional technique is performed during halftoning. The output halftone image data generally can be printed directly, for example by using one printer head for all of the low density inks and a different printer head for all of the high density inks.
As noted above, one example of such multi-level halftoning is simply to print no dot, a light dot or a dark dot for each pixel, based on which is closest to the pixel's input intensity level. However, this technique often does not provide good visual results. In particular, by using the foregoing multi-level halftoning technique in regions where the input intensity level is greater than 127, each pixel generally will be represented by either a low density ink dot or a high density ink dot. Therefore, within those regions ink coverage ordinarily will be equal to 100%. In this situation, it is difficult to perform ink limitation, which is described in more detail below.
Another example of such multi-level halftoning is shown in FIG. 2. Specifically, for each color plane an index, corresponding to a four-position dot grid, is generated for each pixel, depending upon the pixel's input intensity level. Thus, as shown in FIG. 2, if the input intensity level for cyan falls within range 1, Index #0 which corresponds to an empty four-position grid is output. On the other hand, if the input intensity level falls within range 2, Index #1 is output, corresponding to a grid having only a light dot in each of the upper left and lower right corners. Similarly, if the input intensity level falls within range 4, Index #3 is output, corresponding to a grid having dark dots in the upper left and lower right corners and light dots in the lower left and upper right corners.
Although the foregoing technique provides density separation, it is often difficult to obtain high quality printing when using this technique. Specifically, when the dark density ink and light density ink are printed using different printer heads, misalignment of the printer heads can often result in noticeable degradation of image quality.
FIG. 3 illustrates this problem. specifically, FIG. 3A shows the printer output when Index #3 is printed in the ideal situation, i.e., where the two printer heads are perfectly aligned. However, as shown in FIG. 3B, when a small horizontal misalignment occurs, dots begin to overlap and portions of the light dots are obscured, resulting in less ink coverage. The visual effect of this situation is to lower resolution and decrease overall intensity levels. As shown in FIG. 3C, the printed document becomes even further degraded when misalignment becomes more severe.
Another conventional technique is illustrated in FIG. 4. According to this technique, input correction (step S401) and undercolor removal (step S402) are performed as in steps S101 and S102, respectively. After step S402, density separation is performed in step S404. Specifically, in step S404 the four input ulti-value CMYK values are converted to four multi-value amounts, one corresponding to each of the four high density ink (CMYK) quantities, and also are converted to four additional multi-value amounts, one corresponding to each of the low density ink (cmyk) quantities.
Density separation in this technique is ordinarily performed by independently mapping each color component value to a low optical density ink amount and to a high optical density ink amount. A typical conventional mapping is shown in FIG. 5, in which the low density ink is assumed to have a density of one-half of that of the high density ink. As shown in FIG. 5, as the input intensity level increases, the low density ink amount increases linearly and the high density ink amount remains zero until the input intensity level reaches 127, at which point the low density ink amount is 255. Further increases in input intensity amount result in a linear decrease low density ink amount and a linear increase in the high density ink amount until the input intensity level reaches 255, at which point the low density ink amount is zero and the high density ink amount is 255. The foregoing mapping is frequently performed using one or two look-up tables.
Returning again to FIG. 4, upon completion of density separation in step S404, ink adjustment is performed in step S405. Ink adjustment involves making adjustments between the amounts of low and high density inks for each ink color so as to achieve a greater range of printable colors.
Color correction is then performed in step S407. Although the goals are the same as in step S104 described above, the processing in step S407 typically is more cumbersome compared to that in step S104 because 8 values (CMYKcmyk) must be corrected. Moreover, because two values need to be corrected for each color in this technique, often it can be difficult to insure that tones continue to change smoothly after such processing has been completed.
In step S408, output correction is performed based on printer head characteristics.
Finally, in step S409 halftoning is performed in each of the eight color/density planes in order to binarize the ink values for subsequent printing.
While accomplishing density separation, the foregoing conventional processing also has the following problems. First, an additional step of ink adjustment ordinarily is required to be performed, which can require a significant amount of complicated processing. Furthermore, as noted in detail above, performing color correction becomes more cumbersome when two values for each color are to be processed. Finally, in order to insure smooth tonal changes, color correction in this technique generally is required to be specifically tailored to the number of inks, as well as to the ink densities. As a result, changing the number or the densities of the inks used is often difficult with this conventional technique.
Accordingly, what is needed is a more flexible and efficient technique for achieving density separation and ink adjustment.
Moreover, in order to avoid bleeding of ink droplets, it is often desirable to limit the number of ink droplets in each predetermined area, particularly with respect to high intensity levels for which a large number of ink droplets are to be deposited on the recording medium. This type of ink limitation is especially important when more than one ink is used to generate an output color.